Multiple copy electrophotographic device utilizing a charge pattern at the interface of a photoconductive layer and a dielectric layer



I Feb. 25. 1969 T. R. KOEHLER 3,

MULTIPLE COPY ELECTROPHOTOGRAPHIC DEVICE UTILIZING A CHARGE PATTERN AT THE INTERFACE OF A PHOTOCONDUCTIVE LAYER AND A DIELECTRIC LAYER Filed Oct 24, 1965 FIGJ,

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United States Patent 5 Claims ABSTRACT OF THE DISCLOSURE A method for forming a latent electrostatic image of a light pattern on a belt to be used for making multiple copies of the image. The latent image is formed at the interface of a dielectric and a photoconductor so that the image charge is not dissipated upon toning and transfer of the image.

This invention relates to electrophotography in general and more particularly to a device for producing multiple copies wherein the latent charge pattern is formed within either a dielectric or a photoconductor and not at the surface thereof such that no charge dissipation occurs upon toning and transfer of the toned image whereby multiple copies may be made from a single latent electrostatic image.

Electrophotographic machines have been widely used and fully accepted for use in those applications where a single copy or a limited number of copies of a single document are required. This is the so-called convenience type application wherein a great number of copies of a single document are not required and additionally the prime requirement is that of convenience for the user. The throughput of the conventional electrophotographic machine has therefore not been as important a criteria as such factors as color reproducibility, quality output, machine human factors, etc.

There are, however, a large number of applications where the prime consideration is machine throughput. Conventional electrophotographic machines are not capable, however, of high throughput in that they are limited by the photographic response of the input optical system-photoconductor combination. In utilizing a conventional electrophotographic machine, the entire printing process must be repeated Whether one or one hundred copies of a document are being made and due to the limits placed on the process by the input optical photoconductor combination, high throughput is not possible.

In one well known type of electrophotographic machine employing a reusable photoconductor, a photoconductive drum, such as selenium backed by a conductor, is initially charged in the dark by ion deposition by a corona charging unit. The charged area on the drum is then rotated past an optical station where an original document to be copied is imaged by means of a slit optical system onto the surface of the drum to form a latent electrostatic image of the document. This latent electrostatic image is then developed by flowing a tonercarrier mix over it such that the toner particles are attracted to the charged areas to form a visible powder image on the drum. A sheet of paper is then brought into contact with the toned image and ions of appropriate polarity are sprayed onto the back of the sheet of paper to attract the powder image from the drum to the paper. The toned image on the paper is then passed through a fusing unit which heats the toner to a tacky state to cause it to adhere to the paper to provide a final output copy. Problems encountered in employing this type of 3,429,701 Patented Feb. 25, 1969 machine are that abrasive damage to the drum results during removal of residual toner along with filming of the drum thereby necessitating frequent maintenance. Additionally, as in the Electrofax type machine, one of the limiting factors timewise in the reusable photoconductor machine is that of the photo response of the input optical system-photoconductor combination. Again, when making multiple copies on this type of machine, a new latent electrostatic image is formed on the photoconductive drum for each copy to be made. Thus, no time gain per copy is obtained after the initial warmup period of the machine.

From the above, it will be obvious that a high throughput electrophotographic machine should be capable of producing multiple copies of a document without the requisite formation of the latent electrostatic image for each copy. Additionally, the image quality of this latent electrostatic image should not be degraded each time a copy of it is made. Thus, the charge pattern make-up of the latent electrostatic image should not be contacted directly by the developing toner or output media such as paper. Finally, the photoconductor itself should not be contacted directly by the developer to alleviate abrasion of the photoconductor by the toner during toning, transfer of toner to the output media and cleaning of the residual toner from the photoconductive member.

It is therefore an object of the present invention to provide a novel multiple copy electrophotographic method.

Another object of the present invention is to provide a new electrophotographic method wherein multiple copies may be obtained from a single latent electrostatic image.

Another object of the present invention is to provide a new electrophotographic method wherein a latent electrostatic image representative of a document to be reproduced is formed within the photoconductive member and not on the surface thereof such that no charge dissipation occurs during the toning and toner transfer operation.

Another object of the resent invention is to provide a new photographic method for providing latent electrostatic images which when developed will not exhibit the edge effect.

Other and further objects and advantages of the invention will be apparent from the following more particular description of the preferred embodiment of the invention as illustrated in the accompanying drawings in which:

FIG. 1 is a view representative of an apparatus utilizing the hereinafter described novel method;

FIGURE 1A is a view representative of an apparatus utilizing the preferred embodiment described in FIG- URE 2;

FIGS. 2A, B, C, D and E are views illustrating the preferred embodiment of the subject novel electrophotographic method;

FIGS. 3A, B and C are views illustrating a slight alteration of the hereinafter described novel electrophotographic method.

Briefly, in the preferred embodiment, a positive latent electrostatic image is formed in a photoconductor away from the surface thereof through use of the following process steps:

(1) The photoconductor and insulator layers are passed through a charging unit such that a uniform electrostatic charge of equal magnitude but opposite polarity is deposited on each surface;

(2) The photoconductor is then exposed to uniform radiation such that any charge on its surface is dissipated and appears at the interface between the photoconductor and insulator member;

(3) The photoconductor and insulator member are then passed through another charging means wherein a reverse charge as compared to the original charge is placed on the outer surfaces of the two members which results in a charge of opposite polarity from that of the first charge on the outer surface of the photoconductor and a neutral charge on the outer surface of the insulating layer; and

(4) The photoconductor is then exposed to a light pattern from the thing to be copied and residual charge is removed from the surface of the photoconductor.

For a more detailed description of a system wherein the subject novel method may be utilized, refer first to FIG. 1. In FIG. 1 are shown two rotatable drums 1 and 20. The drums are mounted for rotation and rotated by means not shown. Mounted on the drums for transport thereby is a photoconductive belt 2, the exact makeup of which will be later described in more detail in conjunction with FIGURE 3. The drums 1 and 20 rotate the belt in the direction as shown by the arrow 3. In operative association with the belt 2 is a dual corona charging unit designated generally at 4. The dual corona charging unit 4 has a corona unit on each side of the drum. The shield 5 of the left hand corona unit is grounded While its corona generating electrodes 6 are connected to a positive corona generating potential. The shield 7 of the right hand corona unit is likewise grounded while its corona generating electrodes 8 are connected to a negative corona generating potential. A more detailed description of a dual corona charging unit which is suited for utilization in the subject method is contained in US. Patent 2,922,883 to Giaimo entitled Electrostatic Charging Means and Method. Through utilization of the dual corona charging unit 4, uniform equal but opposite charges are placed on the opposite sides of the belt 2.

The portion of the belt which has been charged is then passed from the corona charging means 4 into alignment with an exposure station generally designated at 9. At the exposure station, a document 10 which is to be reproduced is imaged by means of lens 11 onto the previously charged area on the belt 2. Thus, a latent electrostatic image of the drum 10 is formed on the belt. The thus formed latent electrostatic image then passes by a magnetic brush toning station generally designated at 12 where toner is deposited on the belt 2 to develop the latent electrostatic image in a conventional manner. The magnetic brush may be of any conventional type and is shown in FIG. 1 as being a rotatable cylinder 13 having magnets embedded therein which form tufts which are rotated through a toner supply 15. A more detailed description of a magnetic brush which may be used in the system of FIG. 1 is contained in U.S. Patent 2,890,968 to Giaimo entitled Process and Developer Composition Therefor.

The toned image is then moved around drum and is brought into intimate contact with a sheet of output media 16 which may be conventional paper. A supply of paper 17 is shown held in a hopper 18 and the sheets are fed by means not shown in intimate contact past the belt 2 and in operative association with the corona unit generally designated at 19. The corona unit 19 has its shield 21 grounded and its corona generating electrodes 22 connected to a positive corona generating potential. The corona unit 19 is operative to deposit positive ions on the rear of the sheet 16 such that the toner from the belt 2 will be attracted to the sheet 16. This is a conventional toner transfer technique and a more complete description thereof will be found in US. Patent 2,576,047 to Schaffert entitled Method and Apparatus for Printing Electrically. Inside of the drum 20 is shown a conductive mandrel 23 which is grounded. The conductive mandrel 23 provides an electrical field between the corona unit 19 and is such that the positive ions from the corona unit 19 are drawn toward the mandrel and deposited on the sheet 16.

The sheet 16 carrying the toned image is then passed beneath a heat rfixing unit 24 :which is of a conventional type which tackifies the toner and thus, upon cooling,

a lfixed copy of the document 10 is provided which is then fed by means not shown into an output hopper 25.

The belt 2 is then rotated such that the portion thereof which previously carried the toned image passes by the dual corona charging unit generally designated at 26. The dual corona charging unit 26 comprises two corona units which have their shields 27 and 28 grounded and their corona generating electrodes 29 and 30 connected to alternating current corona generating potentials. The shields 27 and 28 in the configuration herein described will be transparent such that light from the flood lamps 31 and 32 can pass through them.

The flood lamps 31, 32 and the dual corona generating unit 26 will be utilized when no more copies of the latent electrostatic images are to be made as will hereinafter become more apparent. Thus, exposure of the belt 2 to the flood lamps 31 and 32 makes the photoconductor conducive and the corona unit 26 operative to neutralize any charge on the pattern such that a new latent electrostatic image can be formed.

The belt then passes a toner cleaning brush 33 which is operative to remove any residual toner from the belt 2. The residual toner will be removed from the belt 2 whether or not the latent electrostatic image has been erased by flood lamps 31 and 32 and corona unit 261 The belt then passes again by the corona unit 4 and the process is then repeated. As previously stated, if multiple copies are desired, the latent electrostatic image will not be erased through use of the flood lamps 31 and 32 and dual corona 26. The latent electrostatic image then would pass through the dual corona charging unit 4 which would then be inoperative and would then pass the exposure station 9 which would also be inoperative back through the toning station 12. The latent electrostatic image would then again be toned and the process previously described repeated to provide another output copy. The final copy would be followed by the erasing of the latent electrostatic image.

As previously described, the system employed a dual corona charger for laying down a uniform equal but opposite charge pattern on the opposite sides of the belt 2. It will, of course, be obvious to those skilled in the art that other means for laying down a uniform equal but opposite polarity charge on the opposite surface of the belt could be utilized.

Thus, for instance, conductive rollers connected to relatively high potentials of opposite polarity could be rolled in contact with the opposite sides of the belt 2 to thereby lay down the required charge on the belt. As previously described, the system shown in FIGURE 1 relates to a belt having a photoconductive layer sandwiched between two dielectric layers as shown in FIG- URE 3.

Refer next to FIGURE 1A wherein there is shown a system which utilizes a two layer belt. That is, a belt as shown in FIGURE 2 consisting of a photoconduetive layer and a dielectric layer. From this figure it is seen that as the belt is passed around drum 1 the belt is charged by a corona unit 50 which places a negative charge on the outer layer of the photoconductive portion of the belt and a positive charge on a dielectric surface of the belt. After passing through station 50, the belt is then uniformly illuminated by means of lamps 55 and 56. As the photoconductor is exposed to this uniform radiation the charge on the surface of the photoconductor travels through the photoconductor and appears at the interface between the photoconductor and the dielectric. Following this uniform radiation, the remaining steps of the process for this system are identical to the process steps described with reference to FIGURE 1 in relation to the three layer system of FIGURE 3.

Refer next to FIG. 2 wherein will be shown a novel electrophotographic method wherein a number of copies may be made from a single latent electrostatic image. The preferred embodiment is shown in FIG. 2 while an alternate embodiment is shown in FIG. 3. As shown in FIG. 2, a sandwich made of a photoconductive layer 35 deposited on a dielectric layer 36 is utilized. The photoconductor 35 may be of any conventional type of photoconductor such as zinc cadmium sulfide, zinc oxide, selenium, etc. The dielectric 36 must have good insulating characteristics and additionally, it should be fairly durable since as will be described, the toner used in developing the electrostatic image is preferably applied to the dielectric.

The charge patterns which result during the five steps of the preferred method are illustrated in FIG. 2. In FIG. 2A, a charge pattern is shown deposited on the surface 37 of the photoconductor and surface '38 of the dielectric. The charges as illustrated are uniform and equal but of opposite sign. The next step in the process is to uniformly illuminate the photoconductor. This results in the charge pattern as illustrated in FIG. 2B. The positive charges which resided on the surface 37 of the photoconductor have now migrated to the interface 39 between the photoconductor and dielectric. The negative charges which had been deposited on the surface 38 of the dielectric 36 remain. In the third step in the process, the sandwich is passed through a dual corona charging unit or conductive rollers, such that a charge of equal but opposite polarity to that of the first charging (2A) is deposited on the surfaces 37 and 38. The results are as shown in FIG. 2C wherein the negative charges which were on the surface 38 are neutralized and a uniform negative charge resides on the surface 37 of the photoconductor. The positive charge pattern which resided at the interface 39 remains unaffected. In the fourth step of the process, the thus prepared sandwich is illuminated by illumination of a document to be copied. Thus, a latent electrostatic image is formed as shown in FIG. 2D. Where light strikes the photoconductor 35, the charges on the surface thereof and the charges resident at the interface 39 neutralize each other while in the dark areas, the charges on the surface of the photoconductor are unaffected. Observations, however, of the electrical field 41 as illustrated in FIG. 2D will show that it remains substantially in the photoconductor 35 and that very little of the electrical field is available for toning. Obviously, if the member 36 is made relatively thin, then some of the electric field would extend above the surface 38. This electric field would, however, exhibit the well known edge effect. A better electric field distribution is provided through the use of the fifth step which is to remove the charge from the surface 37 of the photoconductor such that the latent electrostatic image is formed only at the interface 39. The sandwich of FIGURE 2D is passed between an insulating roller and a conductive roller. The insulated roller is applied to the dielectric material and the conductive roller is applied to surface 37. With the removal of the charge pattern from the surface 37 as shown in FIG. 2E, the electrical field 41 emanating from the charge pattern at the interface 39 is similar to the electrical field associated with a free body in space. Thus, no edge effect is exhibited as illustrated in FIG. 2E and there is a substantial electrical field extending above the surface of the photoconductor 35 and dielectric 36 which may be developed by conventional means.

It will, of course, be obvious to those skilled in the art that the preferred method of developing the latent electrostatic image of FIG. 2B would be with a magnetic brush.

Refer next to FIG. 3 wherein is shown an alternate embodiment of the novel electrophotographic process. In FIG. 3 is shown a three member sandwich comprised of a dielectric 42, a photoconductor 43 and a dielectric 44. The first step of the process is to uniformly charge the surfaces 45 and 46 of the dielectric layers with a uniform charge of equal but opposite polarity. In the second step the thus charged photoconductor dielectric sandwich is illuminated by light from the document to be reproduced and a charge pattern as illustrated in FIG. 3B

is produced with negative charges representing the latent electrostatic image appearing at the interface 47 and positive charges representing the altent electrostatic image appearing at the interface 48. The charges on the surfaces 45 and 46 on the dielectric members are unaffected by this illumination. Observation of the electrical fields associated with the latent electrostatic image will show that the fields 50 remain totally within the photoconductor 43 and none of the electrical field extends above the surfaces 45 and 46 of the dielectrics. Obviously, however, if the dielectrics 45 and 46 were made relatively thin, then some of the electrical field from the latent image would extend above the surface of the dielectric 45 or 46. This field pattern when developed would exhibit the well-known edge effect. In the third step, the latent electrostatic image is rendered useable by removing the charge from one of the surfaces 45 or 46. As illustrated in FIG. 3C, the sandwich has been passed between an insulating roller and conductive roller. The insulated roller was applied to surface 45 while the conductive roller was applied to surface 46. This results in the electrical field from the charge pattern as illustrated in FIG. 3C extending above the surface 46 of the dielectric. The electrical field 50 results due to the fact that the positive charges on the surface 45 and the negative charges appearing at the upper interface effectively neutralize each other and, therefore, the electrical field associated with the positive charges at the lower interface 48 assumes the pattern associated with a free body in space.

It will be obvious that the alternate method as described in association with FIG. 3 produces a negative image whereas the embodiment of FIG. 2 produces a positive image which is usually the more desirable.

In the preferred embodiment a uniform charge pattern of equal but opposite sign was placed on the surface 37 and 38. It will, of course, be obvious to those skilled in the art that instead of using, for instance, a dual corona charging unit to provide this equal but opposite charge pattern that the lower corona unit could be replaced by a metal plate against the surface 38 of the dielectric 36. The steps of the process would be unchanged, however, with the metal plate against the surface 38 of the dielectric, toner would have to be applied to the surface 37 of the photoconductor which would result in abrasion and wear to it.

In the preferred embodiment, a positive latent electrostatic image is formed in a photoconductor away from the surface thereof through use of the following processing steps:

(1) The photoconductor 35 and insulator layers 36 are charged on their outer surfaces 37 and 38, respective.- ly, with a uniform charge pattern of equal magnitude but opposite polarity;

(2) 'Izhe photoconductor 35 is then exposed to uniform radiation such that any charge on its surface 37 is dissipated and appears at the interface 39 between the photoconductor and insulator member;

(3) The photoconductor and insulator member are then passed through another charging means wherein a. reverse charge as compared to the original charge is placed on the outer surfaces 37 and 38 of the two members which results in a charge of opposite polarity from that of the first charge on the outer surface 37 of the photoconductor and a neutral charge on the outer surface 38 of the insulating layer;

(4) The photoconductor is then exposed to a light pattern from the thing to be copied; and

(5) Residual charge is removed from the photoconductive surface 37. This latent image may then be developed by conventional means and then utilized as described in conjunction with FIG. 1.

While the invention has been particularly shown and described with reference to the preferred embodiment thereof, it will be understood by those skilled in the art that various changes in the form and details may be made without departing from the spirit and scope of the invention.

What is claimed is:

1. A method of forming a latent electrostatic image of a light pattern on a belt to be used for making multiple copies of said image comprising the steps of:

(a) charging a belt made with a first layer of photocondnctive material and a second layer of insulating material with a uniform charge of one polarity on said first layer and a uniform charge of the opposite polarity on said second layer;

(b) exposing said first layer to a uniform illumination;

(0) charging said belt with an equal but opposite charge so as to reverse the polarity of the charge that appears on the surfaces of said first and second layers;

( d) exposing said first layer of material to a light pattern to form a latent electrostatic image at the interface of said first and second layers;

(e) removing a residual charge from the surface of said first layer.

2. The method of claim 1 wherein the residual charge from the surface of said first layer is removed by passing said first layer over a conductive roller.

3. A method for forming a latent electrostatic image of a light pattern on a belt to be used for making multiple copies of said image comprising the steps of:

(a) charging a belt made of a first layer of insulating material, a second layer of photoconductive material and a third layer of insulating material with a uniform charge of equal but opposite polarity on the surfaces of the first and third layers of material;

(b) exposing said belt to a light pattern to form a latent electrostatic image at the interface of said first and second layers.

4. The method of claim 3 further comprising the steps of removing the residual charge from the surface of said first layer.

5. The method of claim 4 wherein the residual charge is removed from the surface of said first layer by a conductive roller.

References Cited UNITED STATES PATENTS 2,955,938 11/1960 Steinhilper 96-l NORMAN G. TORCHIN, Primary Examiner.

JOHN C. COOPER, Assistant Examiner. 

